1. Field of the Invention
The present invention relates to a method for determining an acceleration factor for a parallel image acquisition using multiple coil units, for a data acquisition direction of a measurement volume, in magnetic resonance tomography.
2. Description of the Prior Art
Parallel acquisition techniques (PAT) can achieve an acceleration of the pure measurement time associated with pulse sequences in magnetic resonance examinations (MR examinations). Here, use is made of the fact that certain location information can in each case be assigned to measured signals from respective MR antennas based on differing sensitivity profiles. A prerequisite for PAT is thus a number of MR antennas, hereinafter called coil units, respectively with spatially different sensitivity profiles. The output signals of the coil units can be signals from individual coils or combinations of these signals, referred to as modes. What is important is that the large number of sensitivity profiles differs in the direction of the acceleration desired.
Acceleration by means of PAT is possible both within a slice plane of an MR examination in phase encoding direction and in the direction of the slice sequence. The dependencies of measurement record parameters, in particular of an acceleration factor, in connection with PAT can in principle be considered independently of one another for each of the possible directions of measurement. An overall acceleration factor is then produced e.g. from the product of the acceleration factors of the different directions.
Depending on the coil units used, the acceleration factor of a magnetic resonance measurement by means of PAT is a freely selectable, generally user-adjustable, parameter. The specification of an appropriate value for this acceleration factor is extremely complex, requires a great amount of experience and precise knowledge of the currently applicable measurement situation. This situation is determined by the coil units used in each case, their spatial arrangement relative to one another and their possible contributions to the acceleration. For each MR measurement a fresh individual decision has to be reached regarding the acceleration factor, in order to achieve good measurement results in a short measurement time. This procedure, based on a manual input and an individual assessment, for selecting the acceleration factor is time-consuming and poorly reproducible and places high demands on the operator carrying out the MR measurement.